In Power BI, both "GroupBy" and "Summarize" are operations used to create summary table and aggregate data basis certain criteria, yet they have differences between them.

One such explanation is Group By is a function that allows to organize data into groups based on specific criteria. Aggregation can be performed on grouped data. It is useful to segment data based on attributes such as regions, product categories, or customer segments.

On the other hand, Summarize is particularly useful to create new summary tables by aggregating data based on specific columns. It provides flexibility to define multiple summarization operations in a single step and create custom columns based on these summarizations.